1. Field of the Invention
This invention relates to an x-ray tube which is employed in particular in radiology in order to obtain an x-ray beam which can have different directions in space.
2. Description of the Prior Art
This type of x-ray tube is employed for example in diagnostic radiology for scanning a zone to be analyzed or for obtaining at least two x-ray beams having different energy characteristics and/or different angles of incidence on the zone to be analyzed.
A x-ray tube comprises, within a vacuum enclosure, a cathode constituted by a heated filament which emits electrons and by a focusing device which is mounted against the filament and focuses the electrons emitted on an anode which is brought to a positive potential with respect to the cathode. The point of impact of the electron beam on the anode constitutes the x-radiation source in the form of a beam.
In order to produce an angular displacement of the x-ray beam, it is generally proposed to displace the point of impact of the electron beam on the anode by making use of deflecting means. These deflecting means usually consist of magnetic or electrostatic lenses which are placed on the path of the beam or in proximity to said path between the cathode and the anode. Utilization of these lenses calls for not negligible power consumption by reason of the high kinetic energy of the beam electrons which is due to their high velocity as a result of a considerable potential difference between cathode and anode which is greater than one hundred kilovolts.
In French patent No. 2,538,948, there was proposed a scanning x-ray tube in which the focusing device has at least two metallic components which are electrically isolated from each other and from the filament so as to permit their independent polarization with respect to this latter and thus to obtain deflection of the electron beam.
FIG. 1 shows diagrammatically an x-ray tube of the type described in the patent application cited earlier. Within a vacuum enclosure represented in dashed outline by the rectangle 11, said x-ray tube comprises a filament 12, a focusing device 13 mounted against the filament 12 and an anode 14. The filament 12 and the focusing device 13 constitute a cathode C1. The focusing device 13 is constituted by a first metallic component 15 and a second metallic component 16 which are electrically isolated from each other by means of an insulating wall 17 rigidly fixed to an insulating base 18. The metallic components 15 and 16 are placed symmetrically on each side of the filament 12 with respect to a plane of symmetry which is perpendicular to the plane of FIG. 1. This plane of symmetry contains the axis of the filament 12 at right angles to the plane of FIG. 1 and is perpendicular to the base 18. The intersection of said plane of symmetry with the plane of FIG. 1 defines the axis 19 of the electron beam.
When equal voltages are applied to the metallic components 15 and 16, the cathode C1 emits an electron beam F along the axis 19, focusing of said beam being obtained by the geometry of the cathode C1.
In order to obtain a deflection of the electron beam, or in other words in order to give this latter a mean direction which is different from the emission axis 19, it is only necessary to introduce dissymmetry in the electric field produced around the filament 12 by giving different values to the voltages applied to the metallic components 15 and 16; one of these values can be zero but no value must be positive. A beam F' having an axis 19' is thus obtained in the case of a positive potential difference between the component 15 and the component 16; on the other hand, a beam F" having an axis 19" is obtained in the case of a negative potential difference between the component 15 and the component 16.
The x-ray tube which has just been described offers satisfactory deflection performances without requiring the application of unduly high voltages. However, focusing of the beam is not satisfactory for applications in which the x-ray source has to be a point source and the energy distribution of the x-ray beam must be uniformly and symmetrically distributed over its entire cross-section.
In order to overcome these disadvantages, the invention proposes an x-ray tube in which the functions of focusing and deflection are separated spatially at the level of the cathode.